Lung cancer was a very rare disease before cigarette smoking became common in the 20th century. The incidence of lung cancer has increased rapidly, and in Western countries, lung cancer is the most frequent cancer in both men and women. In Korea, lung cancer frequently occurs in men, and its incidence in women is also dramatically increasing. This increase in the incidence of lung cancer is attributable to increases in cigarette smoking, air pollution and industrial pollution and so on.
Even at the present time when medical science has advanced, the 5-year survival rate of cancer patients, particularly solid tumor patients (other than blood cancer patients) is less than 50%, and about ⅔ of all cancer patients are diagnosed at an advanced stage and almost all die within 2 years after cancer diagnosis. Such poor results in cancer therapy are not only the problem of therapeutic methods, but also due to the fact that it is not easy to diagnose cancer at an early stage and to accurately diagnose advanced cancer and to carry out the follow-up of cancer patients after cancer therapy.
Recently, genetic testing methods have actively been attempted to diagnose cancer. Among them, a typical method is to use PCR to determine whether or not the ABL:BCR (Abelson Murine Leukemia Viral Oncogene Homolog: Breakpoint cluster region) fusion gene that is a genetic indicator of leukemia is present in blood. Furthermore, another method has been attempted, in which the presence of genes expressed by cancer cells is detected by RT-PCR and blotting, thereby diagnosing cancer cells present in blood cells. However, this method has shortcomings in that it can be applied only to some cancers, including prostate cancer and melanoma, has a high false positive rate. Also, it is difficult to standardize detection and reading in this method, and its utility is also (Kopreski, M. S. et al., Clin. Cancer Res., 5:1961, 1999; Miyashiro, I. et al., Clin. Chem., 47:505, 2001). In addition, genetic testing using a DNA in serum or plasma has recently been actively attempted. The use of DNA isolated from cancer to analyze cancer-specific gene abnormalities, such as the mutation, deletion and functional loss of oncogenes and tumor-suppressor genes, allows the diagnosis of cancer.
Meanwhile, a method is being attempted in which the presence of cancer cells or oncogenes in the sputum or bronchoalveolar lavage fluid of lung cancer patients is detected by a gene or antibody test (Palmisano, W. A. et al., Cancer Res., 60:5954, 2000; Sueoka, E. et al., Cancer Res., 59:1404, 1999). However, in order to accurately diagnose cancers that involve a large number of gene abnormalities and show various mutations, a method capable of simultaneously analyzing a large number of genes in an accurate and automatic manner is required, but such a method has not yet been established.
Accordingly, methods of diagnosing cancer by measuring DNA methylation have recently been proposed. DNA methylation occurs mainly at cytosines of CpG islands in the promoter region of a specific gene, and thus the binding of a transcription factor is hindered so that the expression of a specific gene is silenced. Thus, analysis of the methylation of the promoter CpG island of tumor-suppressor genes is very helpful in cancer research. An active attempt has been made to analyze the methylation of the promoter CpG island by methods such as methylation-specific PCR (hereinafter, referred to as “MSP”) or automatic base sequencing and to use the analysis results for the diagnosis and screening of cancer.
Although there are disputes about whether the methylation of promoter CpG islands directly induces oncogenesis or causes secondary changes in oncogenesis, it has been confirmed that tumor suppressor genes, DNA repair genes, cell cycle regulator genes and the like in various cancers are hyper-methylationed so that the expression of these genes is silenced. Particularly, it is known that the hypermethylation of the promoter region of a specific gene occurs in the early stage of oncogenesis.
Accordingly, the promoter methylation of tumor-related genes is an important indicator of cancer and can be used in many applications, including the diagnosis and early detection of cancer, the prediction of the risk of oncogenesis, the prediction of the prognosis of cancer, follow-up examination after treatment, and the prediction of a response to anticancer therapy. Indeed, an attempt has recently been actively made to examine the promoter methylation of tumor-related genes in blood, sputum, saliva, feces or urine and to use the examination results for the diagnosis and treatment of various cancers (Esteller, M. et al., Cancer Res., 59:67, 1999; Sanchez-Cespedez, M. et al., Cancer Res., 60:892, 2000; Ahlquist, D. A. et al., Gastroenterol., 119:1219, 2000).
Currently, the diagnosis of lung cancer is possible by various examinations, and if a symptom suspected of lung cancer exists, chest X-ray examination, microscopic examination, video examination, biopsy, examination of metastasis or the like is performed to determine whether the symptom is lung cancer and to determine the degree of progression of lung cancer. However, this detection method requires an expensive system, is costly, has difficulty and is not suitable for the early diagnosis of lung cancer, and in addition, there is difficulty in sampling. Thus, in view of the fact that the 5-year survival rate of stage I lung cancer patients having a tumor size of less than 3 cm reaches about 70%, diagnosing lung cancer at an early stage when the size of the lesion is small is the best method. Accordingly, it is urgently required to develop a detection method which is more efficient than various existing lung cancer detection methods. Namely, it is required to develop a novel lung cancer-specific biomarker which can diagnose lung cancer at an early stage, treat large volumes of samples and has high sensitivity and specificity.
Accordingly, the present inventors filed and received a patent for a microarray and kit for cancer diagnosis including the colon cancer-specific expression-decreased genes LAMA2 (laminin merosin alpha 2), FABP4 (fatty acid binding protein 4), GSTA2 (glutathione S-transferase A2), STMN2 (stathmin-like 2), NR4A2 (nuclear receptor subfamily 4, group A, member 2), DSCR1L1 (down syndrome critical region gene 1-like 1), A2M (alpha-2-macroglobulin) and SEPP1 (selenoprotein P, plasma, 1) (Korean Patent Registration No. 10-0617649).
The present inventors have made many efforts to develop a diagnostic kit capable of effectively diagnosing lung cancer and, as a result, have found that lung cancer and the stage of its progression can be diagnosed by measuring the degree of methylation using the methylated 5′UTR or methylated exon 1 region of PCDHGA12 (GenBank NM_032094) gene, which is specifically methylated in lung cancer cells, as a lung cancer-specific biomarker, thereby completing the present invention.